Northern Ireland's First Minister Peter Robinson has accused Sinn Fein of attempting to blackmail the police over the arrest of Gerry Adams.

The Sinn Fein president has spent a fourth day being questioned by detectives about the 1972 murder of mother-of-10 Jean McConville, who was snatched from her home in front of her children.

On Friday, officers were granted another 48 hours to quiz the republican party leader at Antrim police station. They must now charge or release him by 8pm on Sunday.

Image:Gerry Adams has spent a fourth day being questioned by detectives

Sinn Fein has warned it will review its support for policing if Mr Adams is charged, but Mr Robinson said: "The PSNI must not be the subject of republican bully boy tactics.

"The protest action taken by Sinn Fein is unacceptable in any democratic country operating under the rule of law.

"The publicly conveyed threat to the PSNI delivered by the highest levels of Sinn Fein that they will reassess their attitude to policing if Gerry Adams is charged is a despicable, thuggish attempt to blackmail the PSNI."

Senior Sinn Fein member Gerry Kelly told reporters outside Antrim police station he had seen Mr Adams and that he was being treated well.

"And like myself and all the members of Sinn Fein he believes that the timing of this was political, that the extension of it was political and he's worried about the damage that it might be doing to the image of policing as well and that it's been mishandled in that type of fashion," he said.

Speaking at the unveiling of a mural of Mr Adams in Belfast on Saturday, Deputy First Minister Martin McGuinness alleged some people in the PSNI are "hostile" to the peace process.

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He said: "In my view this is a failed attempt at the replay of the effort in 1978 to charge Gerry Adams with membership of the IRA. That case was based on hearsay, gossip and newspaper articles.

"It failed then and it will fail now.

"Thirty-six years later those within the PSNI who are hostile to the peace process are using the same old dirty tricks. They are deliberately and cynically exploiting the awful killing of Jean McConville and the grief and hurt inflicted on her family."

Sinn Fein's decision to sign up to support the PSNI in 2007 was viewed as a major milestone in the peace process.

"Information became available, allegations Mr Adams was involved in the killing of Jean McConville, the police obviously have to investigate that, they are obviously going to have to speak to Mr Adams at some point, and Mr Adams himself recognised that."

Mr Adams, 65, has been questioned for up to 17 hours a day since his arrest, Sky News understands.

He vehemently denies allegations that he ordered the murder of Ms McConville.

No one has ever been charged with the murder of 37-year-old Ms McConville. Her body was finally discovered in 2003 on a beach 50 miles from her home.